It depends on your length of stay. In general, it is cheapest to stay on a monthly rate (or even a seasonal rate). Just checked one of our local RV parks, and what they charge seems fairly common: The rates there indicate that the daily highest rate at just under $40+tax whereas the monthly rate comes out on a nightly basis to about $16.50+electric.
In general, clubs like Passport America can get you a 50% discount, but that's off the daily rate, not the monthly rate, and your length of stay at that rate is usually restricted to a short period (and often to certain time periods as well). In other words, it's a good source of savings for short-term stays.
What you might consider, however, is an outfit like the Escapees, which runs a few of its own parks at very good rates.
:)
Lynn